1. Field of the Invention
The present invention relates to method for generating a reference signal sequence, and more particularly, to a method for grouping sequences having a variable length corresponding one or multiple of a resource block size, a method for generating a reference signal sequence and a method for generating a reference signal sequence using Zadoff-Chu (ZC) sequence.
2. Discussion of the Related Art
Following explanation is mainly discussed in view of 3GPP LTE system, but the present invention is not limited to this system, and exemplary 3GPP LTE system is only for making those skilled in the art clearly understand the present invention.
There are a lot of sequences used for transmitting signal, but in 3GPP LTE (3rd Generation Partnership Project Long Term Evolution) system, CAZAC (Constant Amplitude Zero Auto-Correlation) sequence forms the basis sequence for transmitting signals. CAZAC sequence can be used to various channels for extracting ID or control information, such as uplink/downlink synchronization channels (SCH) including P-SCH (primary SCH) and S-SCH (Secondary SCH), pilot channel for transmitting reference signal. And, the CAZAC sequence can be used in scrambling.
Two types of CAZAC sequences, i.e., GCL CAZAC sequence and Zadoff-Chu CAZAC sequence are mainly used as the CAZAC sequences. The two types of CAZAC sequences are associated with each other by a conjugate complex relation. That is, the GCL CAZAC sequence can be acquired by conjugate complex calculation for the Zadoff-Chu CAZAC sequence. The Zadoff-Chu CAZAC sequence is given as follows.
                              c          ⁡                      (                                          k                ;                N                            ,              M                        )                          =                              exp            ⁡                          (                                                jπ                  ⁢                                                                          ⁢                                      Mk                    ⁡                                          (                                              k                        +                        1                                            )                                                                      N                            )                                ⁢                                          ⁢                      (                          for              ⁢                                                          ⁢              odd              ⁢                                                          ⁢              N                        )                                              [                  Equation          ⁢                                          ⁢          1                ]                                          c          ⁡                      (                                          k                ;                N                            ,              M                        )                          =                              exp            ⁡                          (                                                jπ                  ⁢                                                                          ⁢                                      Mk                    2                                                  N                            )                                ⁢                                          ⁢                      (                          for              ⁢                                                          ⁢              even              ⁢                                                          ⁢              N                        )                                              [                  Equation          ⁢                                          ⁢          2                ]            where k represents a sequence component index, N represents a length of CAZAC sequence to be generated, and M represents sequence ID or sequence index.
When the Zadoff-Chu CAZAC sequence given by the Equations 1 and 2 and the GCL CAZAC sequence which is a conjugate complex relation with the Zadoff-Chu CAZAC sequence are represented by c(k;N,M), these sequence can have three features as follows.|C(k;N;M)|=1 (for all k,N,M)  [Equation 3]
                                          R                          M              :              N                                ⁡                      (            d            )                          =                  {                                                                      1                  ,                                                                              (                                                            for                      ⁢                                                                                          ⁢                      d                                        =                    0                                    )                                                                                                      0                  ,                                                                              (                                                            for                      ⁢                                                                                          ⁢                      d                                        ≠                    0                                    )                                                                                        [                  Equation          ⁢                                          ⁢          4                ]            RM1,M2;N(d)=p (for all M1, M2 and N)  [Equation 5]
The Equation 3 means that the CAZAC sequence always has a size of 1, and the Equation 4 shows that an auto-correlation function of the CAZAC sequence is expressed by a delta function. In this case, the auto-correlation is based on circular correlation. Also, the Equation 5 shows that a cross-correlation is always a constant.
Among these two kinds of CAZAC sequence, the following explanation is mainly focused on the Zadoff Chu sequence (hereinafter “ZC sequence”).
In the 3GPP LTE system, using this ZC sequence as reference signal sequence, the length of the ZC sequence should be equal to the resource block size. And, not only using one resource block size sequence, but the reference signal sequence having the length corresponding to multiples of resource block size can be used.
For a single-cell environment, the reference signals are transmitted by the localized FDM (Frequency Divisional Multiplexing) method for multiplexing signals from multiple user equipments (UEs). But, for the multi-cell environment, the reference signals are transmitted by the additional CDM (Code Divisional Multiplexing) method for distinguishing the signals from that of the neighboring cells. In this multiplexing, two type of method is possible. One is a CDM method using a ZC sequence having a different root indexes, and the other is a CDM method using a ZC sequence having the same root index (M) and but having differently applied cyclic shift.
When the length of the reference signals using these kinds of ZC sequences is same, the cross correlation values for both of the cases are not large. But, when the reference signals having a difference length came as interference from the neighboring cells and transmitted through the same frequency band or overlapped frequency band, the cross correlation value would be significant.